The present invention relates to power semiconductors and particularly to a highly-reliable package for wide band gap power devices such as silicon carbide (SiC) power device applications and/or combined SiC and silicon power device applications.
Packaging requirements for high switching frequency, high-power device applications require effective heat transfer and removal as well as resistance to stress-related, mechanical or thermal, degradation. Indeed, effective packaging design must strike a delicate balance between these two requirements. Providing greater heat transfer, typically, comes at the expense of increased thermal stress, and vice versa.
Heretofore, to meet these requirements, conventional packaging has included insulated metal substrates, direct-bonded copper packages, ceramic substrates, alumina substrates, and other related heat transfer devices. More recently, the use of carbon nanotubes as bumps for flip-chip application has been proposed.
A conventional insulated-gate bipolar transistor (IGBT) module package is shown in FIG. 5. The module 70 includes at least one semiconductor power device die 72, an electrically-insulating layer 74, a heat-dissipating layer, e.g., a heat spreader 76, and a heat removal device or heat sink 78.
Connecting wires 71, such as copper or aluminum wires, are bonded to input/output pins on the exposed, upper surface of the silicon (Si) power devices 72. An electrically-insulating layer, typically a ceramic such as aluminum oxide (Al2O3), aluminum nitride (AlN) or berylia 74, is soldered to the lower surface of the Si power device die 72, to isolate the devices 72 electrically.
The under side of the insulating layer 74 is soldered to a copper or aluminum heat spreader 76. Typically, metal layers 73 and 75 are provided on top and under sides of the insulating layer 74 to facilitate mechanically coupling the insulating layer 74 to the Si power device die 72 on its top side and to the heat spreader 76 on its under side.
The heat spreader 76 is thermally-coupled to the heat sink 78. A layer of grease 77 often separates the heat spreader 76 from the heat sink 78. Arrow 79 shows the direction of heat transfer for the prior art device 70.
The increased use of Wide Band Gap semiconductors (WBG), such as silicon carbide (SiC) and gallium nitride (GaN), and the anticipated greater use of diamond, especially in power semiconductor applications, can produce smaller, faster, and cheaper semiconductor power packages. Smaller semiconductor power device offer higher power density, which is advantage. However, smaller packages produce higher operating temperatures (T) and higher thermal loading for the power package.
Although, SiC has a higher coefficient of thermal conductivity than Si—making SiC a better choice for heat transfer purposes—it also has a lower coefficient of thermal expansion (CTE). As a result, in comparison with a Si device, a SiC device can produce a higher gradient of thermal expansion between the substrate and the bonding materials. However, higher gradients of thermal expansion produce higher mechanical and thermal stresses.
Advantageously, when compared to pure Si devices, WBG devices support higher switching frequencies. Higher switching frequencies can reduce the required size of passive components, e.g., capacitors and inductors, and can enhance power quality. However, the disadvantages of faster switching speeds include greater power losses due to parasitic and/or stray inductance and parasitic and/or device capacitance.
A large percentage of all electronic device failures are due either to overheating or to mechanical or electrical stresses within the electronic package. Because WBG devices accommodate elevated operating temperatures and because smaller devices having higher power densities produce more heat per area, reliability due to overheating remains a design concern. At very high operating temperatures, wire bonds can delaminate, causing a degradation of performance. Furthermore, stresses resulting from excessive expansion/contraction caused by thermal cycling of the electronics and, more particularly, stresses resulting from excessive expansion/contraction resulting from a mismatch of material properties, e.g., coefficient of thermal expansion (CTE), between adjacent layers in the module or package also remain a design concern. This is especially true at the interface between ceramic insulators having relatively low thermal conductivity and low CTE and heat sinks or heat spreaders or conductive layers having relatively high thermal conductivity and comparatively high CTEs.
Insulated metal substrates are low-cost packages that exhibit good thermal performance. The CTE mismatches within insulated metal substrates, however, is large. Direct-bonded copper (DBC) substrates provide improved CTE matching and good thermal performance. DBC substrates, however, are more costly than insulated metal substrates. Thus, there continues to be a need for new and better packaging technology to provide better thermal matching and improved electrical conductance at reasonable cost.
Accordingly, it would be desirable to provide a highly-reliable, high-speed, thermal-resistant module package for a power device.